Hydraulic booster brake system



Nov. 10, 1953 w. s-rr-:LzER

HYDRAULIC BOOSTER BRAKE SYSTEM 2 Sheets-Sheet l Filed Jan. 9. 1950 INVENTOIL MM...

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Patented Nov. 10, 1953 UNITED STATES PATENT oFFlcE HYDRAULIC BOOSTER BRAKE SYSTEM William Stelzer, Summit, N. J. Application January 9, 1950, Serial No. 137,501

17 Claims. (Cl. 60--54.5)

This invention relates to a hydraulic booster brake system, and more particularly to an improvement over the braking system disclosed in my co-pending application No. 151,119 filed March 22, 1950 where by manual power a small amount of fluid is forced to the wheel cylinder of an automotive vehicle to apply the brakes, and a larger amount of uid is displaced at negligibly low pressure to control a power booster to augment the amount of fluid transmitted to the wheel cylinders. The improvement noted consists of means whereby the manually generated high pressure can be transmitted to the wheel cylinders after the power means are unable to further augment the amount after maximum power has been reached.

In this basic system operating on the new principle which may be termed volume follow-up, a manually operated master cylinder is employed having many elements similar to a conventional two-stage or compound master cylinder. All the known types of two-stage or compound master cylinders are based on a change, i. e., reduction in output volume for a given pedal travel when a certain pressure is reached. The result is the same as a change in pedal ratio which takes place somewhere during the operation. The advantage gained is a reduction in pedal pressure when the hydraulic pressure is high, but thechange felt through the pedal is objectionable.

The object of the present invention is to obtain a similar advantage, but without the change in ratio which would be noticeable in the application of the brake pedal. The advantage resides in the continuance of delivery of hydraulic iluid to the wheel. cylinders by manual power after the power means fail to further augment the volume oi' fluid transmitted to the wheel cylinders, the reaction on the brake pedal remaining proportionate to the wheel cylinder pressure, ySince a high braking pressure may be produced even after failure of the power means to augment the volume transmitted to the wheel cylinders, it is obvious that the braking system may be constructed with a smaller booster mechanism, thus reducing the cost.

Another object is to provide means for accommodating the control fluid after the booster mechanism runs out of power, and means to prevent the increased pressure in the wheel cylinders to react on the booster when the pressure produced by the latter is too low.

A further object is to safeguard rsuficient output of hydraulic fluid in case of power failure'by forcing the control fluid displaced by the master 2 cylinder to be transmitted under higher pressure to the wheel cylinders, until a pre-determined pressure is reached.

y A still further object is the control of the residual pressure, where the pressure of the control iluid is completely relieved when the system is in the released position.

Other objects and advantages of the invention will be apparent from the following detailed description consideredin connection with the accompanying drawings submitted for the purpose of illustration and not intended to define the scope of the invention, reference being had for that purpose to the subjoined claims. In the drawings, wherein similar reference characters refer to similar parts throughout the several views: f

Fig. 1 is a cross-sectional view of the power operated ybooster mechanism;

Fig. 2, a cross-sectional elevation of the manually operated master cylinder, used in connection with the booster shown in Fig. 1;

Fig. 3, a section taken on line 3-3 of Fig. 1;

Fig. 4, a top end View of the valve actuating lever shown in Fig. l;

Fig. `5, a cross-sectional elevation of a special master cylinder of modied construction embodying the novel elements otherwise shown in Fig. l;

Fig. 6, a fragmentary sectional view of the booster used in connection with the master cylinder shown in Fig. 5; and Fig. 7, a fragmentary sectional view similar to Fig. 5 but showing the elements in a position assumed during operation. y

`Beioreexplaining in detail the present invention it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practised or carried out in various ways. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not limitation. y l Describing the invention now in detail, and referring to Fig; 1,k there is shown a power operated booster mechanism having a cylinder or body comprising a bore II in which slides a piston I2 provided with a seal I3 and secured to a piston rod 'I4 passing through bearings I5 and I6 and seals I1 and I8 which are all securely retainedat the end of body I0. The other extremity of piston rod I4 carries a reinforced diaphragm plate I9 and a diaphragm 20 Whose outer beadA ZI 3 is clamped between cover 22 and casing 23 secured to body I0.

Power piston or diaphragm plate |9, diaphragm 20, cover 22, and casing 23 constitute the motor mechanism of the booster. Cover 22 and casing 23 are secured together in any suitable manner. the method shown having been explained in detail in the co-pending application referred to. As a source of power for the motor mechanism the differential pressure between a source of low pressure 24 and a source of higher pressure 25 is used. Thus 24 may represent a source of vacuum such as the intake manifold of an internal combustion engine, connected through line 26 to chamber 21 of the motor mechanism. Body I also has a bore 28 in which slides a sealed control piston 29 havinga piston .rod or stem 39 extending through bore 3| to unseat ball valve 32 urged toward its seat by a light spring 33 retained by a plug 34. The other end of the control piston has a rod extension 35 with a snap ring or shoulder 36 engaging a conical return spring 31 urging piston 29 towards the left into the starting position. Rod 35 also serves to support and operate valve lever 38 held against ring 36 by a spring 39 which is considerably stiffer than spring 31. The valve lever or plate 38 has raised portions 40 along its pivotal axis, as shown in Fig. 4; They facilitate the rocking action of the lever during actuation of vacuum valve 4I and atmosphere valve 42. The latter is urged into a closed position by a spring 43 guided by a stud 44 secured to a casing 23. Valve 4I has its stem pivotally secured to lever 38 as a small clearance is permitted between nut 45 and lever 38 so that the valve can find its proper position when seated. Due to return spring 31 valve 4| is unseated when the brakes are in the released position, whereby both sides of the motor mechanism, chambers 21 and 46, are equalized and exposed to low pressure, as they are connected together through passage 41, chamber 48, and line 49. In this position the power piston of the motor mechanism is held against cover 22 by a return spring 59. Actuation of the valves to cause energization of the motor mechanism is effected by hydraulic pressure in chamber generated by the manually operated master cylinder shown in Fig. 2, the hydraulic pressure being transmitted through line 53 through a passage in the form of a groove 54 in extension 39 to chamber 5|. It will be noted that groove 54 does not extend the entire length of extension 30. This is to provide a suiiicient passage for operation of piston 29 to control the valves, but to effect a restriction during overtravel of piston 29, a slow flow of fluid being then permitted through the clearance between extension 38 and bore 3|. Valve lever 39 is permitted a certain travel with piston 29 until it rests against stud 44 and low pressure valve 4| is closed, but piston 29 is capable of continuing the movement in response to pressure in chamberr5| by compressing springs 39 and 31 until rod 35 stops against casing 23. In order to provide the restriction only in one direction, I incorporate a check valve 55 which allows piston 29 to return quickly by allowing uid to pass through hole 56 into line 53. 'I'he latter is also in communication with primary chamber 51 of the booster through check valve 32, chamber 58, and passage 59. The secondary chamber 60 is connected through line 6I to the wheel cylinders 62 which actuate the brakes.

The master cylinder shown in Fig. 2 is similar 4 to that shown in my co-pending application Ser. No. 151,119. It comprises a cylinder body 63 having a reservoir 64 and a bore 65 in which slides a piston 66 actuated by a push rod 61 and connected to the usual brake pedal, not shown. One extension of the piston forms a high pressure plunger 68 passing through bearing 69 and seal 10 into chamber 1| formed by a cylinder or hollow plug 12 secured to cylinder body 63 and retaining a sleeve or ring 13 which secures seal 18 in place and also serves as a stop for valve 14 so that it is unseated when the master cylinder is in the released position, as shown. Chamber contains the control fluid. Piston 66, which has a seal 16 retained by a return spring 11, displaces iluid from chamber 15 to primary chamber 51 of the booster and to chamber 5| to induce energization of the motor mechanism. The space between seal 16 and primary seal 19 is open to reservoir 64 to prevent any possibility of air being drawn into the system. A passage 13 leads from this space through plunger 68 so that when valve 14 is open chamber 1| is in communication with the reservoir whereby the pressure in chamber 1| is completely relieved. A residual valve 89 yieldingly held against the bottom of the bore of plug 12 by spring 8| assures a residual pressure in the wheel cylinders which are connected to plug 12 by means of line 92, the latter being also connected to line 6| so that the pressures in chambers 60 and 1| are the same. Spring 8| serves the dual purpose of tending to seat the residual valve as well as valve 14, the

latter seating when plunger 63 is moved toward the left during a brake application. The displacement of iiuid from chamber 1| to the wheel cylinders is permitted by the check valve built into the residual valve, this construction being conventional, it need not be further described. In the return stroke, the piston is arrested in the proper position by stop ring 83.

In the modification shown in Figs. 5, 6, and 'l the elements which produce the desired results are located in the master cylinder instead of the booster. Accordingly, in the booster shown in Fig. 6 valves 32 and 55, as well as spring 39, are eliminated, but the motor mechanism and its valve arrangement are the same as shown in Figs. l and 3.

Cylinder body 99 has a bore 9| in which slides a modied control piston 92 having a central guide pin 93 and a rod extension 94 carrying valve actuating lever 38 which is connected to the valves of the motor mechanism in a similar manner as vshown in Fig. 3. Spring 31 urges the control piston toward the left tc assume the oi position, where the motor mechanism is deenergized. In this embodiment the control piston moves only a very short distance, which is enough to provide sutllcient valve movement. Therefore, rod 94 extends to within a. short distance of casing 23 so that it is arrested by the latter at the end of the stroke preferably before lever 38 touches stud 44. Rod 94 is stepped to provide a shoulder against which lever 38 rests and on which it may rock. To enable the lever to rock freely, snap ring 36 is located to allow a small clearance between it and raised portion 49 on which the lever rocks. The hydraulic fluid in chamber communicates with chamber 51 through passage 96 and with the master cylinder through line 91.

The modied master cylinder shown in Fig. 5 comprises a cylinder body 99 having a bore 99 in which slides a compound piston consisting of 'a low pressure piston |00 anda high kpressure plunger or piston securedk together and actuated by' a conventional push rod |02. The low pressure piston has two seals |03 and |94, the space between which is permanently open to reservoir through hole |03. yPlunger I0| extends through a bearing |01 and seal |08 to enter into a high pressure chamber |09 connected to the wheel cylinders 62 through line IIO with a residual valve III interposed seated-against end plug II2. An auxiliary piston I|3 also slides in bore 99 but it furthermore is capable of sliding on plunger IOI in yielding to a high pressure in chamber I I4 by compressing spring II5 which ordinarily presses piston II3 and its seal |I6 against collar |I1 rmly secured to plunger IOI. A return spring II8 urges the compound piston into its released or starting position to rest against a xed stop plate |31. In this position shown chamber |99 is relieved of pressure as it is in communication with reservoir |05, needle Valve I|9 being unseated from piston |20 sliding in a central bore in plunger IDI, one end of piston forming a valve |2| whose object is to control the relief of fluid from chamber |22 to the reservoir, for which purpose a hole is provided in plunger IfJI to lead from chamber |22 to the clearance around the stem of valve |2| and another hole to connect valve chamber |23 with the space between seals |93 and |04. A valve spring |24 urges valve |2| into a closed position and also opposes the hydraulic pressure acting on piston |20 from chamber |09 when valve |I9 has closed the central passage |25 which extends through piston |20 and valve I2 I. Spring |24 is proportioned in relation to the area of piston |20 so that valve |2| opens at a predetermined pressure which must be sufficiently high for a safe brake application'but below the maximum output pressure of the booster mechanism. The ypurpose of piston |20 and valve |2| is merely to assure suflicient output kof iiuid by the master cylinder in case of power failure. Valve I I9 has secured to it a disc or plate |26 to serve as a stop against sleeve |21 used to retain seal |08. Disc |26 also engages valve spring |28 which urges valves I|9 and I|| into a seated position. A ball valve |29 urged into a seated position by a light spring |30 is arranged `in the cylinder body. It is the equivalent of valve 32 in Fig. 1 as its function is to prevent the return of iluid from chamber 51 after the 'booster kis unable to match the manually generated pressure. A piston I3I provided with a trip rod to engage ball |29, slides in bore |32 and is subject to the hydraulic pressure from chamber II4 through passage |33, A long rod |34 part of piston I3I extends through the reservoir to slide in plug |35, a spring |36 urging the piston toward the left to oppose the hydraulic pressure transmitted from chamber I I4- and to open Valve |29. The spring is so proportioned that it yields only when the hydraulic pressure in chamber II4 exceeds the normal operating pressure necessary for the operation of the valves of the motor mechanism.

Describing now the operation of the booster and master cylinder shown in Figs. 1 and 2 and assuming that they are in the released position as shown on the drawing, depression of the brake pedal by the operator causes push rod 61 and piston 66 with its plunger 68 to move toward the left to displace uid from chambers 15 and 1|. A very short movement is requiredbefore valve 14 is closed so that pressure can be generated in chamber 1| and the fluid transmitted through line 82 to the wheel cylinders to apply the brakes. The fluid displaced from chamber 15 is transmitted through line 53 to chamber 5| to act on piston 29 which in turn compresses spring 31 and pushes plate 38. Due tetheresistance of spring 43 lever 38 rocks about 'ball 42 toi'lrst close valve 4| to separate .chamberv 43 from 21. After valve 4| is closed, further movement of lever 38 is possible only by compressing spring 43 to allow pressure valve 42 toopen, whereby air from pressure source 25 is transmitted through chamber 48 and connection 49 to chamber 46 to energize the motor mechanism and to urge piston I2 of the booster towards the left, displacing fluid from chamber to be transmitted under pressure through line 6I to the wheel cylinders to assist in applying the brakes. The iluid transmitted from chamber 15 of the master cylinder not only enters into chamber 5|, but may also iiow into chamber 51 through passage 59. Thus the volume of control uid transmitted from chamber 15 determines the booster output from chamber 50, because an increase in iluid transmitted to chamber 51 would produce a build-up in pressure and cause piston 29 to yield to urge the motor mechanism to increase the power and thereby step up the movement of piston I2 toward the left. On the other hand a decrease in volume yof fluid transmitted to primary or fluid receiving chamber 51 would result in a reduction in pressure and a consequent return of piston 29 toward the released position to reduce the power of the motor mechanism and to slow down or reduce the output from chamber 60. In actual operation the pressure diierences of the control fluid while the booster is in operation are very small and the pressure is so low that the manual effort to produce it is negligible. The effort to actuate piston 68 is reduced in proportion to the amount of fluid transmitted by it to the wheel cylinders. The total volume of fluid received by the latter is equivalent to the output from chamberv 1| of the master cylinder and the displacement from chamber 15 plus the amount of fluid displaced by piston rod I4, or it may be stated that the output from chamber 60 is equivalent to the displacement from chamber 15 plus the iluid displaced by piston rod I4. Accordingly the output of the booster is greater than the amount of iiuid it receives from the master cylinder. This is an advantage which will become apparent when considering the next stage of operation when the booster runs out of power and the only iluid transmitted to the wheel cylinders is from chamber 1I. If 25 represents a source of atmospheric pressure, and 24 a Vacuum, it is obvious that the power of the motor mechanisms cannot be increased indenitely. It is limited by the diameter of the diaphragm. When atmospheric pressure is reached in chamber 46 the power cannot be further increased. This is usually termed power run-out. supposing now that this occurs, and that the operator increases the pressure on the brake pedal to continue the travel of piston and plunger 68 toward the left, a higher pressure is produced in chamber 1I but simultaneously the pressure in chamber 15 is built up because piston I2 fails to follow in' response to the Valve action. yConsequently piston 29 continues toward the right and cornpresses springs 39 and 31. Groove 54 of stem 30 enters totally into bore 3| so that a restriction is introduced which retards the ow of fluid intor 7 chamber' Il, only passageV being provided by the clearance around stem 30' in bore 3l. After sprhg 39 starts to compress, valve n becomes seated.' so that the hydraulic fluid from chamber l1 is prevented from backing up to return to chamber 15 or into 51. 1f this action occurs; too oerlyr fluid can still now from line 53 into cham'- ber It as. ball` 12l then acts as' a check valve. with valve 32 closed, and the booster run out of pmi', the only' discharge is now from chamber 1| by manual power. No change has taken M in the master' cylinder', the reaction on the br'allzeA pedal is still proportional tothe hy'- #allc pressure in the wheel cylinders, and the fluid from chamber 15 under stiliy comparatively low pressure as it flows' into chamber 5i. Thus the flidd transmitted to the' wheel cylinders is' of l reduced amount. Since at this stage the brake sho are fully expanded any further application is mostly a matter of increased pressure wimout any appreciable increase in displacelent so that-only' a very small movement' is required to produce an increased' pressure. Thus the. travel of piston 29 until rod 35 h-its' casing 23 is adequate to accommodate the` fluidf dispMed trom' chamber 51 into chamber 5t. In the retractile' movement,v when the brake pedal is' released and' piston $0 and' plunger $8 return toward the: starting position, the pressure in chamber 15 becomes reduced, so that by virtue of springs 31 and le piston 29 is moved toward the left and fluid from chamber flows back to chamber 18; This is facilitated by check' valve which allows the-fluid' to 'by-pass the restrictien around stem 3U s'o' that-piston- 29 can return quickly. Whenvalve 32 is opened again by stem litho-hydraulic pressure in chamber 51 is again approximately'the same as' it was when the valve closed, andk it is the same as in chamber so that practically no effort is required to unseat balli n. After this theV booster is again in operation as though no change had ever taken place. A continued release of the brake peda1 and consequent reduction in pressure in chamber T5 calmes plstcn- 29 to retract further toward the left' in' response to spring 31' whereby valve 42 rst closes; du'e to spring 43', and then valve #I opens tol release the air from chamber I6! via connection-C8, chamber 48, passage 41, andi chambe't'lr toward the source of low pressure; Helped by the pressure in chamber lill',v the booster now returns toward the releasedl orV starting position in a' true' follow-upfwith the' piston ofithe' master cylinder: A's piston 66` and plunger 68A reach the starting or released' position, the disc shaped part of valve 1( comes to rest against sleeve F3 tov open'A valve T4, so'- that chamber 11|` is againl in communic'ationv with themaster cylinder. Any pressine inthe control circuit is relieved byv pas'- sage of fluid: from chamber 15 past' the inner lip of seal 1l into chamber 11. Chamber and wheel' cylinders 62 are relieved of pressureA only until residual? pressure is reached, residual valve UU tended by spring 81 preventing any further release'of fl'uidf from line 82 to the master cylin d8?.

Operation of the system illustrated' in` Figs. 5'; e; and '1, will' nowbe described'. Figs, 5 and 6 show' the master cylinder and' booster ini theireleased' or starting position. Depression of` the brake pedal by the operatormoves pushro'd In', pinnen l and plunger HH toward' the. left, the Istit!" entering into chamber I09'to' displace fluid from there to be transmitted to the wheel; cyl@ inderonto apply the brakes; In vtheinitial move- 8 ment needle valve H9 E seated on phton l" to shut of! the passage from high pressure chamber l0! to the reservoir. Piston H3 moves in unison with plunger 10| as it is held against collar H1 by the sti spring H5. The uid displaced from chamber IM is transmitted through passage l" and line 91 to chamber 95 to` act on piston l! which is forced to move to the right by compressing sprlng' 31 and actuating lever Il to operate' the' valves of the m'ofo'r' mechanism as al ready described. The fluid is also transmitted through passage 96 into chamber l1 of the booster. Asv the motor mechanism is energiaed due to the valve action induced by the' pressure in chamber 95, piston t2 moves toward the left to' displace uid from chamber 60 to beV trm mitted to the wheel cylinders to apply the brakes. The same volume follow-up action takes place as already described before in reference to Plus.- l and 2.v 'I'he ilui'd from control chamber I I4 isv ditpliaced into chamber 51, and any increased pressure in the latter due to a relative advance ofA the master'cylinder piston is immediately translated into increased energization of the motor mechalmm t in chamber 51', or any decreased pressure due to a retardation of the' master cylinder piston is translated into decreased power of the motor mechanism. After a pre-determined pressure is attained which should be suillciently high' to assure a' saiev brake application, piston |10' yields tothe pressure in chamber' m and moves toward the right by compressing spring |24 and unseating va1ve` f2 I' until the stem` of the' latter resto against the bottom of chamber 123, valve I'I Q'having moved with piston lland' remaining seated. Chamber |22 is now open to the'reservoir. After the power run-out takes place, i. e., where the force exerted' by the moto'r mechanism through piston rod I4 is' unableito'overc'ome the hydraulic pressure' in chamber 6l acting on piston' I2'. the pressure of the controlV fluid is increased As soon as the hydraulic' pressure is'slightly' higher than the'pressure necessary to move piston` I2 for valve action', piston lit yields by compressing spring i, thus closing' valve' Into prevent ree turn of fluid from chamber 61. Further movement of piston no and plunger |n| toward me left producesi an increased pressure in chamber I'N. The amount of fluld displaced is' ver'y'small as the' brakes are fullyapplied so that it i'sn'iostly a matter of' pressure' and` not movement. uringtliis slightmovement the pressure in chamber Ifll' is prevented from buildlngup as piston HI yields and moves away from collar I'I1' and com'- pressesspring H8. The position is illustrated in Fig; 7'. Fluid' is displaced from chamber I!! past valve 12| to reservoir IN. Spring IIB is an element of the construction equivalent to spring 39 of'Pig'. 1, andpis'ton I I3 maybe compared with piston 23,Y the latter,I however', serving a double duty by' alsoY performing' the function of piston 92'. While'th'e' hydraulic pressure inthe wheel cylinders is thus increased, andthe fluid in cham-v ber 61 is locked' it is' apparent that the pressure of the hydraulic fluid acting on piston 9'2 isv increased. 'I'he end of piston rod is therefore pressed against casing 1I, stud 44 in this embodiment serving only as a guide for spring and not as a stop' for valve lever 38. If the operat'or releases the brake pedal to start piston Il!! and plunger 10i on' their retractil'e movement toward the' starting position. piston III is all lowed to'move toward collar I I'1` by force of spring H15.- iluid then being drawn back into chamber HZ either pastl valve Ill or past the outer lip of seal |03. After piston ||3 and seal IIB are up against collar ||1 to assume their original position, any further retractile movement of the compound piston reduces the pressure in chamber ||4 and immediately allows piston |3| to be moved toward the left by spring |36 to unseat ball |29. During the retractile movement described the hydraulic pressure in the wheel cylinders has been gradually reduced to the pressure that existed before valve |29 closed, and consequently the pressure in chamber 51 was also reduced to a pressure which existed before. As the compound piston of the master cylinder continues further towards the released position, the pressure in chamber I |4 becomes reduced so that the lower pressure acting on control piston 92 allows the latter to move toward the left and to thereby operate the valves to reduce the power of the motor mechanism whereby the retractile movement of the booster is started, to follow the retractile movement of the master cylinder. After the hydraulic pressure in chamber |09 has become lower, piston |20 has moved again toward the left by force of spring |24 to close valve |2|. Before piston comes to rest against stop ring |31, disc |25 of valve ||9 is arrested by sleeve |21 and valve ||9 becomes unseated whereby chamber |09 is in communication with resesvoir |05.

If a brake application is made in case of a power failure, Where the booster is not capable of augmenting the output of fluid by the master cylinder, it is necessary that the fluid displaced from chamber ||l| is also transmitted to the wheel cylinders. The fluid displaced from chamber ||4 is forced into chamber |09 past the inner lip of seal |08, or past the outer lip of seal I3. Piston I3 is unable to move away from collar |1 because the outlet from chamber |22 is blocked since valve |2| is closed. It opens only later, when the brakes are fully applied and the hydraulic pressure is high. Due to the greater manual effort necessary it can safely be assumed that such higher pressure would rarely be produced, but if the pressure is actually reached where valve |2| opens, the master cylinder will act as a two stage device and will convert to the second stage by relieving the pressure in chamber |22 and consequently also in chamber ||4 as spring I compresses, so that the reaction on the brake pedal will be reduced. When the brake pedal is released and piston |90 and plunger Hl! return toward the starting position, fluid is drawn by suction from the reservoir into chambers |22 and H. It is able to pass past the outer lips of seals |03 and HS, a suilicient clearance existing between the pistons and bore 99. The main Volume of the fluid returning from the wheel cylinders is not released until the master cylinder piston has reached the starting position where valve ||9 is lifted so that the excess fluid is released through the central bore in plunger 0| to replenish reservoir |85. While it may be objectionable that the brakes are fully released only after the brake pedal has reached the off position, the condition would exist only in an emergency and would not aiect the safety of the brakes. In considering the operation of the construction shown in Figs. 1 and 2 in case of power failure, a similar action takes place as just described, except that in this embodiment plunger |26 and valve |2l are omitted. In its place the restriction around stem 39 is introduced. Its effect is that if the brake pedal is depressed very rapidly so that a considerable pressure is suddenly produced in the control circuit, the premature overtravel of piston 29 is prevented since the restriction acts like a damper. This is also effective when there is no power failure but the motor mechanism cannot follow rapidly enough due to an extremelyl rapid application of the brake pedal.

Since the function and the essential elements of the two different embodiments are the same, it appears evident that the invention does not depend on the location of the devices which lock the booster after the power run-out and accommodate the control iluid displaced from the master cylinder. The 'essential elements are rst, means (32 or |29 and 29 or |3I) to prevent the release of fluid from'the primary chamber of the booster (5l) or in general, to prevent the flow of fluid from ther wheel cylinders to the booster after the latter has run out of power, second, yieldingmeans (||3 or 29) to accommodate the vcontrol iluid displaced from the master cylinder or to provide for the relief of its pressure after the booster has run out of power, and, third, means y to prevent the sudden relief of pressure of the control fluid displaced by the master cylinder (|20, |2I, around stern in case of lag of power of the motor mechanism or a too rapid application of the brake pedal.

I claim:

i. In a braking 'system having wheel cylinders to apply the brakes, a manually operated high pressure fluid displacing device to transmit fluid to said wheel cylinders to apply the brakes, a

power operated booster comprising Iluid displacj ing means adapted to transmit iluid to said wheel cylinders to augment the amount of iluid transmitted by said manually operated high pressure fluid displacing device, a manually operated low pressure fluid displacing device adapted to operate in unison with said ilrst device, meansresponsive to the volume of fluid displaced by said low pressure fluid displacing device to control the power of said booster to transmit a proportionate amount of fluid to said wheel cylinders, said means responsive to the volume of lluid having an expansible chamber to receive iluid from said low pressure fluid displacing device when said booster is unable to transmit more fluid to said Wheel cylinders due to power runout, resilient means tending to reduce the volume of said expansible chamber, and a valve closed by a predetermined expansion of said expansible chamber to check the return of fluid from said wheel cylinders to said fluid displacing means of said booster.

-2. In a braking system having wheel cylinders to apply the brakes, a manually operated high pressure fluid displacing device adapted to transmit fluid to said wheel cylinders, a manually operated low pressure iluid displacing device adapted to work in unison with said rst device, a power operated booster having fluid displacing and receiving means adapted to receive the iluid displaced by said low pressure fluid displacing device and to transmit a proportionate volume under pressure to said wheel cylinders to augment the amount transmitted by said high pressure fluid displacing device, said booster having an expansible chamber connected to independently receive fluid from said low pressure fluid displacing device after power runout of said booster, resilient means urging to lcontract said expansible chamber, and a check valve operatively connected with said expansible chamber |23, or the restriction or clearance*r 1 l to close after a pre-determined expansion of said expansible chamber to check the dow ot duid from said wheel cylinders to said duid displacing and receiving means of said booster aiter the latter has run out of power.

3. In a braking system having wheel cylinders to apply the brakes, a manually operated high pressure duid displacing device adapted to transmit duid to said wheel cylinders, a manually operated low pressure duid displacing device adapted to work in unison with said drst device, a power operated booster having duid pressure receiving and transmitting means adapted to receive the duid displaced by said low pressure duid displacing device and to transmit a proportionate volume under pressure to said wheel cylinders to augment the amount transmitted by said high pressure duid displacing device, means responsive to the power run-out of the booster to block the return of duid from the wheel cylinders to said duid pressure receiving and transmitting means of booster while the latter has insudlcient power, means for relieving the duid displaced by said low pressure duid displacing device while said booster is unable to transmit duid due to power run-out and retarding means to prevent a too sudden relief of pressure of the duid displaced by said low pressure duid displacing device.

4. In a braking system having wheel cylinders to apply the brakes, a pair of simultaneously manually operable duid displacing devices one of which is adapted to transmit a small amount of duid under higher pressure directly to said wheel cylinders, a booster comprising a power operated duid displacing mechanism connected to transmit duid under pressure to said wheel cylinders to augment the amount transmitted by said one device, said booster having an expansible chamber adapted to receive control duid from the other of said duid displacing devices, the intake of control duid being proportionate to the output of fluid of said duid displacing mechanism. means responsive to the pressure of said control duid to increase the power oi said booster to step up the output of said. duid displacing mechanism, and means responsive to an increased and predetermined pressure of said control duid to prevent the return of fluid from said Wheel cylinders to said duid displacing mechanism of booster.

5. The construction as claimed in claim 4, and resiliently yielding means adapted to prevent the excessive build-up of pressure of the control duid while said booster is unable to assist said master cylinder during power run-out.

6. The construction as claimed in claim 4, resiliently yieldable means adapted to prevent the excessive build-up of pressure ot said control duid while said booster is unable to assist said master cylinder during power run-out, and damping means to prevent a sudden relief of Pressure i.' the control fluid.

'7. The construction as claimed in claim 4, resiliently yieldable means adapted to prevent the excessive build-up of pressure of said control duid while said booster is unable to assist said master cylinder during power run-out, and pressure sensitive means responsive to the hydraulic pressure in said wheel cylinders adapted to prevent the relief of pressure of said control duid before a predetermined pressure is reached in said wheel cylinders.

8. In a braking system having Wheel cylinders to apply the brakes, a master cylinder having a pair of simultaneously manually operable duid displacing devices one of which is adapted to transmit a small amount of duid directly to said wheel cylinders, the other displacing control duid, a booster having a power operated duid displacing chamber to transmit duid under pressure to said wheel cylinders to augment the amount transmitted by said one device, a duid receiving chamber operated in unison with said fluid displacing chamber arranged to receive control duid from the other of said devices at a rate proportionate to the discharge from said duid displacing chamber, pressure sensitive means responsive to the pressure of the control duid displaced by said other device to increase the power of said booster and thereby increase the output of said duid displacing chamber, said pressure sensitive means being adapted to receive control duid from said master cylinder when said booster is arrested due to power run-out, a check valve to prevent the flow of duid from said fluid receiving chamber to said pressure sensitive means and to said other duid displacing device of said master cylinder after power run-out of said booster, and spring biased means opposed by the pressure of said control duid transmitted from said master cylinder to open said check valve.

9. The construction according to claim 8, and a restricted passage arranged to slow down the dow of control duid to said pressure sensitive means.

l0. The construction according to claim 9, and a check valve arranged to provide a passage for a quick return of control fluid from said pressure sensitive means to said other duid displacing device and to said duid receiving chamber.

11. The construction according to claim 9 where said pressure sensitive means comprises resilient means to bias said pressure sensitive means into a released position where said booster is deenergized, and additional resilient means arranged to yieldingly oppose the overtravel of said pressure sensitive means in response to a higher control duid pressure.

12'. In a braking system having wheel cylinders to apply the brakes, a manually operated master cylinder having a control piston to transmit control fluid under very low pressure and a high pressure piston of relatively small cross-sectional area to transmit duid to said wheel cylinders, said two pistons being arranged to move in unison, a booster comprising a power operated duid displacing mechanism arranged to transmit duid to said wheel cylinders to augment the amount delivered by said piston of small cross-sectional area, a chamber increasing proportionately with the displacement of duid from said duid displacing mechanism to receive control duid from said master cylinder, means responsive to the pressure of said control duid to control the power oi said booster to increase the output of said duid displacing mechanism, spring operated means to bias said means for controlling the power of said booster into a position to deenergize said booster, a check valve arranged to prevent the return of control duid to said master cylinder, resilient means arranged to open said check valve, and means responsive to the hydraulic pressure of said control duid transmitted by said master cylinder to oppose said resilient means to urge said check valve into a closed position.

13. The construction as claimed in claim l2, and means to provide for the relief of the control duid in said master cylinder when a predetermined pressure is reached which is higher than the hydraulic pressure necessary to urge said means responsive to the pressure of the control iiuid to increase the power of said booster.

14. The construction according to claim 12, where said control piston is slideable relative to said piston of small cross-sectional area, and a spring arranged to yieldingly urge said control piston to travel in unison with Said piston of small cross-sectional area.

15. The construction according to claim 14, and means responsive to the hydraulic pressure of said Wheel cylinders to prevent said control piston from yielding to said control iluid before a certain pressure is reached in said wheel cylinders.

16. The construction -as claimed in claim 12, and means to relieve the hydraulic pressure of the system When said master cylinder is in the released position.

17. In Va braking system having Wheel cylinders to apply the brakes, a manually operated master cylinder having a control piston to transmit control fluid under very low pressure and a high pressure piston of small cross-sectional area to transmit fluid to said wheel cylinders, an auxiliary piston exposed to said control fluid and tended by spring means to resiliently yield to the pressure of said control fluid, a hydraulic fluid reservoir, a chamber containing hydraulic fluid behind said auxiliary piston, a valve to open a passage from said chamber to said reservoir,

14 resilient means urging to close said valve, means responsive to the hydraulic pressure of said wheel cylinders to open said valve when a certain pressure is reached to make it possible for said auxiliary piston to yield to a higher pressure of said control fluid, a booster comprising a power operated iiuid displacing mechanism, means for receiving the control fluid displaced by said control piston, means to control said booster to transmit an amount of fluid to said Wheel cylinders proportional to the volume of control uid received from said master cyllinder, and a check valve arranged to .prevent the return of control fluid to said master cylinder, a spring arranged to open said check valve, and means responsive to the hydraulic pressure of said control fluid to oppose said -spring to urge said check valve into a closed position.

WILLIAM ISTELZER..

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,834,368 Arbuckle l Dec. 1, 1931 2,322,063 ySchnell June 15, 1943 2,328,637 Freeman Sept. 7, 1943 2,352,357 Almond June 27, 1944 2,353,755 Price July 18, 1944 

